Liquid crystal display (LCD) panels used in PC monitors, TVs, and even portable DVD players use discharge lamps as backlight devices.
Commonly used discharge lamps include cold cathode fluorescent lamps (CCFLs) and external electrode fluorescent lamps (EEFLs). A DC to AC switching inverter is commonly used to power these lamps at very high AC voltage. Usually the DC voltage is chopped by power switches to produce an oscillating voltage waveform and then a transformer and filter components are used to produce a near sinusoidal waveform with sufficient amplitude. CCFLs are usually driven by AC signals having frequencies that range from 50 to 100 kilohertz.
The power switches may be bipolar junction transistors (BJT) or field effect transistors (MOSFETs). Also, the transistors may be discrete or integrated into the same package as the control circuitry for the DC to AC converter. Since resistive components tend to dissipate power and reduce the overall efficiency of a circuit, a typical harmonic filter for a DC to AC converter employs inductive and capacitive components that are selected to minimize power loss. A second-order resonant filter formed with inductive and capacitive components is referred to as a “tank” circuit, since the tank stores energy at a particular frequency. Higher order resonant filters may also be adopted.
The average life of a CCFL depends on several aspects of its operating environment. For example, driving the CCFL at a higher power level than its rating reduces the useful life of the lamp. Also, driving the CCFL with an AC signal that has a high crest factor can cause premature failure of the lamp. The crest factor is the ratio of the peak current to the average current that flows through the CCFL. On the other hand, it is known that driving a CCFL with a relatively high frequency square-shaped AC signal maximizes the useful life of the lamp. However, since the square shape of an AC signal may cause significant interference with other circuits disposed in the vicinity of the driving circuitry, the lamp is typically driven with an AC signal that has a less than optimal shape, such as a sine-shaped AC signal.
Double-ended (full-bridge and push-pull) inverter topologies are popular in driving today's discharge lamps because they offer symmetrical voltage and current drive on both positive and negative cycles. The resulting lamp current is sinusoidal and has a low crest factor. These topologies are very suitable for applications with a wide DC input voltage range.
Single-ended inverters are often considered for low-power and cost-sensitive applications. The new single-ended inverters proposed in applications Ser. No. 10/850,351 can efficiently drive discharge lamps at low crest factor and offers much lower voltage stress than the traditional single ended inverter, is thus very attractive for the low power and cost-sensitive applications.
To achieve good regulation on both lamp current and open lamp voltage, it usually requires multiple complicate regulation loops to control the switching frequency and the duty cycle of the switching AC waveforms that are generated from the switching devices in the above mentioned inverter topologies. This invention proposes a unique and simple control scheme. The following discussion is based on the new single ended topology. However, the same control scheme can be applied to other topologies, including full bridge, half bridge and push-pull.